Films and membranes of polymers insoluble in polar solvents are obtainable at present by methods such as hot lamination, pressing and/or sintering, a typical example of which is the formation of sheets of polytetrafluoroethylene (PTFE). Such methods are somewhat sophisticated and require the availability of expensive apparatus. A variety of sulfonated and sulfonable polymers suitable for the preparation of ion-selective membranes or of porous diaphragms with enhanced ion-selective properties have been recently developed in the field of electrodialysis and electrolysis. For example, in electrolytic cells, a typical example of which are cells for the electrolysis of alkali metal chlorides, the membrane or diaphragm must be resistant to an environment that is particularly hostile to polymeric compounds, such as the exposure to a very high pH and to chlorine. In general, hydrocarbon polymers are totally unsatisfactory for this use and copolymers of a halogenated monomer and a monomer containing polar groups are usually suggested. However, such polymers are generally insoluble in polar and non-polar solvents and their formation into very thin sheets or their application onto inert support materials such as asbestos, for example, by means of impregnation, has proved to be difficult.